this seems to be a common fault over time, it comes from the battery being voltage to frame.
as some machines have a sensor to frame check it must be as low as possible to remove the fault.
this comes with water spillage and over filling batteries.
ma worst case was full voltage to frame, easily seen by corrosion build up around battery and case.
the only way to reduce this is to remove the battery from the machine, take to workshop and steam clean between case and battery.
also sprinkle bicarbonate of soda over all the battery if acid is present you will see fizzing, if no fizzing then let battery dry and put on charge.
wait for a day then conduct voltage to frame check.
if it is still higher than 15volts to frame then continue the cleaning process.
my worst battery took three days to clear after which the forklift was back to full use.
Hope you find this helpful.